A fuel cell system has recently been thrown as a high-efficiency energy conversion apparatus into the limelight.
Some types of the fuel system have been operated or under research and development. Of these systems, a proton-exchange membrane fuel cell system utilizing proton as electrolyte has a compact structure, and in addition, ensures high power density and can be operated in the form of a simple system, thus receiving attention in use not only as stationary distributed batteries, but also as power supply sources for space, vehicles and home use.
Such a highly expected fuel cell system, especially the proton-exchange membrane fuel cell system has a structure comprising a fuel cell body 1, a fuel reformer 2 and a purging line, as shown in FIG. 13.
The fuel cell body 1 is provided with a membrane electrode composite body in which a proton-conductive solid polymer membrane 4c is for example held between gas diffusion electrodes 4 with catalytic layers. Separators 5a, 5b serving as a collector, which have gas supply grooves and are formed of material having a low gas permeability, are alternatively placed in a stacked state on both the outer sides of the membrane electrode composite body.
The gas diffusion electrode 4 is provided on its one side with a fuel electrode 4a, and on the other side with an oxidizing electrode (an air electrode) 4b. Fuel gas mainly containing hydrogen and air are separately supplied into these electrodes through the gas supply grooves of the separators 5a, 5b. 
At this stage, an oxygen ion generated from the oxidizing electrode 4b passes through the solid polymer membrane 4c to react with hydrogen in the fuel electrode 4a, thus emitting an electron. Such an emitted electron is utilized as a direct current electric power.
The fuel reformer 2, which generates fuel gas mainly containing hydrogen from hydrocarbon fuel such as city gas, reforms the hydrocarbon fuel into the fuel gas mainly containing hydrogen by making a chemical reaction of the city gas, from which sulfur contained therein has been removed utilizing a hydrogenation desulfurization, with the use of a catalyst, for example adding steam into it.
The purging line 3, which is provided with an inert gas (for example, nitrogen) supply equipment 6 and an inert gas valve 7, is connected to a downstream side of a fuel valve 10 of a fuel supply line 8. The purging line 3 acts to purge condensed water and unreacted hydrogen gas, which was generated during production of the above-mentioned direct current electric power and still remained, even after termination of operation, in the fuel cell body 1, the fuel reformer 2 and the fuel supply line 8, with the use of the inert gas supplied from the inert gas supply equipment 6 through the inert gas valve 7, and discharged exhaust outside from the system through an exhaust pipe 9.
In the conventional fuel cell system, the purging operation has been carried out in order to cope with an explosion accident due to the unreacted hydrogen and corrosion by the condensed water, in this manner.
In the conventional fuel cell system as shown in FIG. 13, the purging operation has been carried out, when the operation is halted, to purge combustible gas and condensed water outside from the system, with the use of inert gas such as nitrogen, taking into consideration explosibility due to the residual combustible gas caused by unreacted substance from the fuel supply line, corrosiveness of equipment and damage to catalyst with which the equipment is covered, which are caused by the residual condensed water, and deteriorated activity due to oxidation of the catalyst caused by oxygen gas.
However, the inert gas (for example, nitrogen gas) supply equipment has a complicated structure in system, a large size, an increased cost and an increased running cost, and further requires many incidental facilities, thus causing many kinds of inconveniences and problems.
The fuel cell system, which requires the nitrogen supply equipment, has hardly been accepted for home use, thus constituting an obstacle to application or expansion to the home.
An object of the present invention, which was made in view of these circumstances, is to provide a fuel cell system, which include a purging apparatus for reliably removing combustible gas and condensed water, which remain in the system, in a simple structure and an easy operative manner, and a purging method therefore.